Benzo thiophenes, hydroxy tautomerism

Unlike the hydroxy derivatives, no tautomerism tends to exist in the simple 2- and 3-thiophenethiols <1977ACB198>, nor in benzo[. ]thiophene-3-thiol <1970JC2431>. 

Benzo[6]thiophene, 2-acetyl-3-hydroxy-synthesis, 4, 892 Benzo[6]thiophene, 2-acyl-synthesis, 4, 918 Benzo[6]thiophene, 3-acyl-synthesis, 4, 918- 19 Benzo[6]thiophene, acylamino-synthesis, 4, 815 Benzo[6]thiophene, alkenyl-synthesis, 4, 917 Benzo[6]thiophene, 2-alkoxy-synthesis, 4, 929 Benzo[6]thiophene, 3-alkoxy-synthesis, 4, 929 Benzo[6]thiophene, 4-alkoxy-synthesis, 4, 930 Benzo[6]thiophene, 2-alkyl-synthesis, 4, 877-878 Benzo[6]thiophene, 2-alkylthio-synthesis, 4, 931 Benzo[6]thiophene, 2-amino-diazotization, 4, 810 reactivity, 4, 797 stability, 4, 810 synthesis, 4, 869, 924-925 tautomerism, 4, 38 Benzo[6]thiophene, 3-amino-cycloaddition reactions, 4, 68 synthesis, 4, 109, 881, 925 Benzo[6]thiophene, 4-amino-synthesis, 4, 925 Benzo[6]thiophene, 5-amino-synthesis, 4, 925 Benzo[6]thiophene, 7-amino-synthesis, 4, 925 Benzo[6]thiophene, 3-t-amyl-synthesis, 4, 915 Benzo[6]thiophene, 2-aryl-synthesis, 4, 881... 

Equilibrium and rate constants for the keto-enol tautomerization of 3-hydroxy-indoles and -pyrroles are collected in Table 32 (86TL3275). The pyrroles ketonize substantially (103-104 times) faster than their sulfur or oxygen analogues, and faster still than the benzo-fused systems, indole, benzofuran, and benzothiophene. The rate of ketonization of the hydroxy-thiophenes and -benzothiophenes in acetonitrile-water (9 1) is as follows 2-hydroxybenzo[b]thiophene > 2,5-dihydroxythiophene > 2-hydroxythiophene > 3-hydroxybenzo[/ Jthiophene > 3-hydroxythiophene. 3-Hydroxythiophene does not ketonize readily in the above solvent system, but in 1 1 acetonitrile-water, it ketonizes 6.5 times slower than 2-hydroxythiophene (87PAC1577). 

Thioindoxyl 2-substituted synthesis, 4, 892 synthesis, 4, 880, 894, 927 tautomerism, 4, 728 Benzo[6]thiophene, 4-hydroxy-synthesis, 4, 905, 928 Benzo[i>]thiophene, 5-hydroxy-synthesis, 4, 928 Benzo[i>]thiophene, 6-hydroxy-synthesis, 4, 928 Benzo[6]thiophene, 7-hydroxy-synthesis, 4, 928 Benzo[f>]thiophene, 2-iodo-synthesis, 4, 934 Benzo[f>]thiophene, 3-iodo-synthesis, 4, 934 Benzo[6]thiophene, 2-mercapto-synthesis, 4, 931 tautomerism, 4, 38 Benzo[6]thiophene, 3-mercapto-synthesis, 4, 931... 

In the benzo[A] and benzol/] heterocycles 220 and 221, where the loss of ring resonance energy on tautomerism is much less than in the nonannulated heterocycles 217, the oxo tautomers 222 and 223 are energetically preferred. 2-Hydroxyindole 220 (X = NH) exists exclusively as the oxo tautomer 222 (X = NH) (oxindole). The hydroxy form of 2-hydroxybenzo[A]thiophene 220 (X = S) is detectable by NMR spectroscopy when its trimethylsilyl ether precursor is... 

The effect of solvent on the equilibrium constants has been studied for the 3-hydroxybenzo[/ ]thiophenes <1986TL3275, 1989JA5346>. The enol is much more stable in hydrogen-bonding solvents than in aptotic solvents. Thus, while enol 213 is barely detected in 060, CCR, or CDCI3, it is predominant in CsDsN and DMSO-i/6. For acid-catalyzed ketonization, the 3-hydroxy monocyclic compounds tautomerize 10-10 times faster than their benzo analogues, presumably via a mesomeric effect due to the lone pair on the heteroatom. 

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